In Vivo Characterization Of Skin Mechanical-spectroscopic Properties And Its Application In The Study Of Facial Expressions

In-depth study of skin mechanics and spectroscopic properties will promote the rapid developments in many fields,including surgery,expression recognition,electronic skin,tactile sensing,and robotics.The mechanical properties and spectroscopic properties of human skin not only enable it to respond to and perceive external changes,but also to convey information to the outside world through selfdriven deformation and color changes.Therefore,based on the deformation response under the external load of the body skin and the internal load of the body,the mechanical properties of the skin were systematically studied by means of indentation,friction and expression-drive.The mechanical properties include nonlinearity,anisotropy,friction and buckling behavior.Moreover,the deformation-spectroscopic coupling behavior of the skin was then quantitatively described,and its application in expression and emotion recognition is further studied.For the mechanical behavior of in vivo skin under indentation load,the hyperelastic and anisotropic mechanical properties of multilayer skin were characterized by theoretical,experimental and finite element method.Firstly,this study introduces two kinds of indentation devices developed by our team to measure the mechanical response of in vivo skin.Secondly,considering the nonlinear mechanical behavior of soft tissue,an indentation model with finite thickness is established.Based on the traditional indentation device and CT technology and theoretical model,the macroscopic mechanical properties of soft tissue(including skin and muscle)in different regions of the face are characterized.Thirdly,this study analyzes the reasons for the differences of elastic modulus in each region from the perspective of anatomy,and expounds the importance of characterizing mechanical properties based on layered structure.In order to study the mechanical properties of the layered structure of skin,the three-dimensional reconstruction of the soft tissues including dermis,subcutaneous tissue and muscle is carried out.Finally,a finite element model which can simulate the real structure,boundary conditions and loading model is established.Combined experiment result with finite element method,a three-dimensional characterization method considering the multi-layer structure,hyperelasticity and anisotropic mechanical behavior of skin is proposed.Aiming to the deformation response of human skin during friction,this study studied the friction behavior of bionic skin and human skin by using theoretical,experimental and finite element methods respectively.This study realizes the prediction of the critical value of macro sliding based on partial data in the static friction stage.Firstly,the force displacement curve and contact image in the whole friction process are measured by visual indentation device,and an image processing method and shape function are proposed to quantitatively determine the contact radius and contact area.Based on this experiment,the empirical formula of the evolution law of the contact area with the change of tangential force is given.Secondly,combined with DIC technology,three-dimensional deformation field of contact interface in the process of friction is measured.Then,the evolution law of slip ratio and bond ratio in contact area with the increase of tangential force is further analyzed.Finally,the sliding friction theory considering deformation and the static friction theory which can predict static friction are proposed,respectively.Using these theories,the friction behavior of bionic skin and in vivo skin can be characterized.In conclusion,this study lays a theoretical support for understanding the skin perception and grasping regulation function.Due to the movement,contraction and relaxation of muscles,the skin will deform under the body’s own factors,even without any external force load.This deformation appear in face is facial expression.In order to quantitatively describe the deformation of the skin under its own factors,this study drives the facial skin to deform through the expression,and measures the deformation field under six basic expressions based on the DIC method.It is found that the maximum compression area(the area corresponding to the minimum principal strain of the facial skin)and its corresponding direction under the expression-drive are basically consistent with the location and texture of wrinkles.On this basis,the deformation transmission and flexion of skin under muscle movement,contraction and relaxation are simulated by finite element method,and the main factors affecting the generation of wrinkles are further analyzed.In vivo skin is often under stress,so this study carries out theoretical and experimental research on the spectral characteristics of skin under deformation.Firstly,through the in-depth study of the tissue structure and spectral characteristics of skin,a theoretical model of multi-layer skin deformation physiology spectroscopy coupling is proposed.Secondly,combined with hyperspectral imaging technology,the changes of spectral reflectance under the two factors of skin deformation and physiological changes are analyzed.The corresponding characteristic bands are further compared and optimized,and the characteristic parameters are defined.Finally,by analyzing the difference of characteristic parameters between spontaneous expression and artificial expression,it is proved that the characteristic parameters can effectively judge whether the subjects have emotional experience.In conclusion,this study lays a new method for emotional perception,realistic robot and simulation animation.